1. Field of the Invention
This invention relates to an image forming apparatus, and more specifically relates to an image forming apparatus which can reproduce a halftone image with high quality.
2. Description of the Related Art
Known non-impact printers referred to as light beam printers are coming into wide use because these printers have a high resolution and are quiet. In such known recording apparatuses, methods generally referred to as the dither method, the error dispersion method and the pulse-width-modulated method are adopted for reproducing halftone images.
In order to reproduce a faithful halftone density by utilizing a density level signal input to the recording apparatus, the printer characteristic, the so-called gamma (.gamma.) characteristic (ideal value: .gamma.=1) is corrected. That is, various controls are performed for making the relation between the input density level signal and the density level of a reproduced halftone image linear.
However, even if a gamma correction is performed for making the relation linear by such controls, a set development characteristic of the printer may vary because of changes in environmental conditions (temperature, humidity, etc.) or an increase of the number of printed sheets, so that the linearity of the relation cannot be maintained.
Accordingly, in the known printer, it has been considered for a controller to correct the variance of the gamma characteristic caused by the variance of environment conditions by changing a gamma conversion table, such as a dither table, for reproducing a halftone image.
However, if such changing of the gamma conversion table is performed by the controller by predicting the change in printing density based on a parameter using environment conditions or the number of printed sheets, an error in gamma correction may be big.
Meanwhile, in the case where the printer receives a modulated signal and turns on/off an image exposure device according to the modulated signal, the potential number of different gamma conversion tables may be increased remarkably, because each printer has different development characteristics.
Further, in the case where a photoconductor, a charger, developing device, and a cleaning device, etc., are detachable relative to the main body of the recording apparatus, that is, a so-called process cartridge, the individual difference of the process cartridge must be considered.
Accordingly, a large-capacity memory is required as the gamma conversion table because the number of different gamma conversion tables is increased, which increases the cost of the apparatus.
When such known electrophotographic image forming apparatus is used for a long time, the sensitivity characteristic of a photoconductor drum is deteriorated, whereby the whole output image is brightened, or a thin portion or a solid portion cannot be reproduced faithfully. Even if the apparatus is not used for a long time, the quality of the output image is deteriorated because of the variance of environment conditions or the dispersion of electrostatic charge, a photosensitive drum, a developing device, or image exposure amount.
In order to mitigate the above described problems, it has been proposed to control the surface electrical potential of the photosensitive drum automatically so as to obtain a uniform potential, or to control the developing bias of the developing device so as to obtain a uniform density of a solid image formed on the photosensitive drum. For example, U.S. Pat. No. 4,872,035 describes an apparatus in which primary charger control is based on solid image portions while lamp control is based bright image portions.
However, such controls are not effective in producing accurate halftone image. Even if the above-described controls are used in such apparatus, it is insufficient to obtain a stabilized image with high quality. In the case where the surface electrical potential or the solid image are controlled, there is the defect that the reproduced halftone image is varied.
A known laser beam printer capable of halftone density printing is structured in such a way that it inputs pulse-width-modulated image data, corresponding to each density, sent from a host computer, and carries out the halftone density printing based on the input image data.
Explanations of an operation for the halftone density printing will be described below with reference to FIGS. 29 and 30.
FIG. 29 is a block diagram showing an arrangement of a known image forming apparatus.
FIG. 30 is a timing chart for signals between a host computer and a printer.
When a command signal, such as a signal for setting printing conditions, output from a host computer 91 shown in FIG. 29 is transmitted on a command/status signal line 93, a printer 92 transmits a reply complying with the command signal as a status signal. Then, the host computer 91 judges the status signal, and transmits a video signal 94 representing image data and a video clock signal 95 representing an image clock to the printer 92 when the status signal instructs the host computer 91 that printer 92 is ready to print.
A semiconductor laser 99 in the printer 92 emits a laser beam when a laser driver 98 is driven in accordance with the video signal 94. The laser beam is irradiated to form an electrostatic latent image on a photosensitive drum by a polygonal mirror (not shown). Then, a toner adheres to the electrostatic latent image, and a visualized toner image is transferred to a fed printing sheet. And then, printing is completed by fixing the transferred toner image.
FIG. 30 shows a timing chart for the signals 94, 95 described above.
As described above, the video signal 94, which is pulse-width-modulated in the host computer 91 so as to represent an image density with its pulse-width, is transmitted to the printer 92 in synchronism with a rising edge of the video clock signal 95. The printer 92 drives the laser driver 98 according to the pulse-width of the video signal 94 so as to execute the halftone density printing. That is, the halftone density printing is performed by controlling an irradiation time of the semiconductor laser 99. In the case where the pulse-width of the video signal 94 is long, the printing density is darkened. Meanwhile, in the case where the pulse-width of the video signal is short, the printing density is brightened.
However, the known image forming apparatus has the following defects.
For example, a 300 DPI (printing density of 300 dots per one inch) printer uses video clock signals having an approximate frequency of 2 MHz, which represents a period of 500 ns. In the case where the printer prints halftone density having sixteen gradations, it is required to have an ability that can divide one dot into sixteen elements.
Thus, pulse signals having a period of 31.25 ns (500 ns.div.16=31.25 ns) are transmitted as video signals to the printer from a host computer through a cable.
Accordingly, when a thin cable is used to transmit the pulse signals, there is a defect that the pulse-width of the video signals varies according to various conditions, such as, kind or length of the cable, temperature or humidity surrounding the printer, or dispersion of electronic parts of the printer. Thus, the printing density cannot be stabilized.